1. Field of the Invention
The present invention relates to integrated optic modulators and particularly to an apparatus and method for controlling the polarization state of light propagated through a non-polarization-preserving fiber to a remotely-located integrated optic modulator so that the polarization state of the light arriving at the input of the integrated optic modulator is maintained in the particular state required for optimum operation of the integrated optic modulator.
2. Description of the Prior Art
Most active integrated optic modulators require a stable predefined input state of polarization (SOP) to operate with optimum efficiency. As a result, a high birefringence polarization-preserving fiber is generally used to provide light with that required stable SOP to the input of an integrated optic modulator. This use is especially significant if the integrated optic modulator is used in a remote sensing or a communications system where remote interrogation of the modulator may occur through a very long fiber link (i.e., from hundreds of meters to several kilometers in length).
A high birefringence fiber is much more expensive than conventional low birefringence fiber. To allow the use of the less expensive low birefringence fibers in the above-named systems, polarization-independent modulators have been developed and used. These polarization-independent modulators use two-electrode structures in order to equally modulate each of the TE and TM polarization modes of the modulator. Since the geometry for a two-electrode structure is more complicated than that for a one-polarization or polarization-sensitive modulator, higher modulation voltages are required, i.e., for a given input voltage, the modulation efficiency is reduced.